388 Field, Forest, and Wayside Flowers 
the ends of its slenderest rootlets. Nature has taken 
care that the water sucked in by the roots shall not 
be evaporated and lost as it goes up through trunk 
and branches to the thirsty little shoots at top. 
So the year-old twigs have a tough skin, which is 
nothing more nor less than a thin sheet of cork, 
while older branches are encased by a thicker cork- 
covering, which lies, as a rule, below the surface. 
Be it thin or thick it is perfectly water-proof. 
The peel of a potato is nothing more nor less 
than a layer of cork-cells, and, by observing the 
quickness with which pared potatoes ‘‘dry out,”’ 
we realize how effective even a thin cork-covering 
can be in preventing the transpiration of vegetable 
moisture. 
The life-giving juices of the tree can not get 
through the cork-layers of the bark to nourish the 
outermost tissues of the trunk and branches. So 
all these parts of the tree which lie outside the 
cork-layer dry up, shrivel, crack apart, and at last 
flake off and fall to the ground. 
These dried and drying tissues may include cells 
of many sorts and sizes, which in their younger 
days served various uses in the tree’s domestic 
economy. But now we speak of them all together 
oe 
as the ‘‘ outer bark.’ 
